The transformation of semi-conductor wafer disks into integrated circuit chips often involves many steps where the disks are repeatedly processed, stored and transported. The wafers must be transported from workstation to workstation and from facility to facility. Wafer disks are brittle and are easily damaged by physical shock. Also, build-up and discharge of static charges in the vicinity of semiconductor wafers can be catastrophic. Due to the delicate nature of the wafers and their extreme value, it is vital that they are properly protected throughout these procedures from contaminates, and physical and electrical damage.
Specialized carriers or containers are used for handling, storing, and shipping wafers. Such devices normally hold the wafers in axially aligned arrays with, for example, twenty-five wafers in an array. A principal component of the containers is a means for supporting the wafers during handling to protect against physical damage from shock and vibration. This wafer support means may be provided with a path to ground for static dissipation through a machine interface on the bottom of the container. Such containers and support means are known and disclosed, for example, in U.S. Pat. Nos. 5,788,082; 6,010,008; 6,216,874; and 6,267,245, all owned by the owner of this invention, these patents being fully incorporated herein by reference. Such containers are known in the semiconductor processing industry as front opening unified pods or FOUPS.
While these specialized containers have allowed for more efficient automated handling of wafers with less damage, a significant problem has been in precisely locating the wafer disks within the wafer container so as to allow for accurate automated handling of the disks in a production process. Wafer supports and their attaching devices must be designed with sufficient rigidity to firmly and precisely support and locate the wafers when the container is in use. Since the processing of wafer disks is generally automated, it is essential for the container to precisely align the wafer disks according to the specifications of the processing equipment being used. The tolerances available to the container are generally very tight for proper interaction between the processing equipment and the wafer disks. The wafer carrier manufacturing industry constantly strives to design carriers with improved tolerances for better assurance of accurate carrier-equipment alignment.
Generally, wafer containers include machine interface portions, including a guide plate or kinematic coupling and a robotic lifting flange, for moving and properly orienting the carrier with respect to carrier interface portions provided by the processing equipment. Machine interface portions, particularly the kinematic coupling, are often used as a reference point for specifying the relative location of wafer disks for automated processing. The walls of the container, especially the top of the container which is connected to a robotic lifting flange, are susceptible to flexing and distortion when carrying the weight of the container. This flexing and distortional movement is transmitted to the wafer supports through their attachments. Stress and undesired shifting in the position of the wafer supports in the container is the result, increasing the potential for carrier-equipment tolerance mismatch and physical damage to the wafers.
Typically, wafer supports are rigidly attached to the wafer container. Most commonly, threaded fasteners are inserted through an opening in the container wall and threaded into the wafer supports. A problem, however, results from the fact that a small amount of clearance must be allowed between the wafer support and the container walls in order to allow insertion and positioning of the wafer support during assembly of the container. This problem is exacerbated by the variations in dimension and warping of the container enclosure and wafer support sometimes encountered in the injection molding process. These variations make it difficult to manufacture a wafer support with sufficient precision to fit all individual enclosures with a fixed amount of clearance. Often, it is not possible to completely take up the clearance when fastening the wafer support in place, and as a result, the required accuracy for disk positioning is not achieved.
Another problem is presented by differing thermal expansion coefficients of the wafer container shell and the wafer supports. Typically, a wafer container shell is made from polycarbonate material, which generally has a thermal expansion coefficient of about 68×10−6 in./in./C°. Wafer supports, however, are often made of PEEK which has a thermal expansion coefficient of about 42×10−6 in./in./C°, or of carbon fiber filled polycarbonate or carbon fiber filled PEEK, which have a thermal expansion coefficient of about 20×10−6 in./in./C°. Wafer containers are subjected to washing operations, typically at 70–80 C°. With rigidly attached wafer supports, the much larger thermal expansion coefficient of the shell material with respect to the wafer support material results in stresses in both the wafer supports and the shell. Over time, failure of the components and dimensional creep can result.
Thus, what is needed is a wafer container having wafer supports that provide rigid support and precise positioning of the wafer disks for automated processing. At the same time, the wafer support must be isolated from flexing of portions of the enclosure, and ideally maintain an electrical path to ground. Also, the wafer support should be capable of installation in a container without the need to allow excessive clearance for positioning of the support.